In a QAM symbol transmission system of the type disclosed, a transmitted data symbol is represented by both "I" and "Q" quadrature components which modulate respective quadrature phased carriers. Each symbol may comprise several bits, and the number of symbols dictates the type of QAM system, i.e., 16-QAM, 32-QAM, etc. Each symbol is mapped (assigned) to a prescribed location in a four-quadrant grid-like constellation using a look-up table (e.g., a ROM). A prescribed number of symbols occupy assigned areas in each quadrant. In a 32-QAM system, each quadrant of the constellation contains eight symbols at prescribed coordinates with respect to quadrature I and Q axes. Certain symbol bits designate the constellation quadrant in which a symbol is located, and certain bits designate the particular coordinate in that quadrant assigned to the symbol. QAM systems of this general type are well known.
White and Raychaudhuri et al. describe aspects of a system wherein a television signal representing high definition image information is transmitted using two QAM carriers frequency multiplexed in a standard 6 MHz television transmission baseband. One of the carriers conveys high priority information, while the other carrier conveys (relatively lower) standard priority information. The high priority (HP) information is the information needed to create a viewable image, although less than a perfect image, and is conveyed with significantly more power than the standard priority (SP) information, which is the remaining information. The high priority information exhibits a narrow bandwidth compared to the standard priority information, and is therefore much less prone to corruption by the transmission channel. The HP carrier is located in that portion of the frequency spectrum of a television transmission channel, e.g., an NTSC channel, which is normally occupied by the vestigial sideband of a standard NTSC television signal. This portion of the signal is normally significantly attenuated by the Nyquist filters of standard receivers, so that HDTV signals with this transmission format will not introduce co-channel interference.
A straightforward approach to encoding such a dual QAM signal is to use two parallel paths for encoding and modulating two QAM signals independently. After the two encoded QAM signals have been converted from digital to analog format, two frequency translators merge the encoded QAM signals into a composite dual QAM signal with carriers situated at appropriate spectral locations. In accordance with the principles of the present invention, an illustrative system encodes a prioritized multiple carrier QAM HDTV signal so as to reduce circuit complexity and production costs, e.g., by reducing overall circuit size and integrated circuit surface area in particular, without compromising the quality of an encoded QAM signal.